Travis Python 2.6

This increases the Travis build environment to include testing against Python 2.6 and 2.7 both with numpy 1.7.1 and 1.6.2.

In the process of doing this I noticed a 2.6 related bug in diff_rot, so I fixed it.

While Travis is not there to test every combination of packages does anyone have any suggestions for other combinations they think are worth testing for?